Back in 1988 as my wife and I were planning a longish tour on our just-being-built tandem, we went into a Marmot store in Berkeley. They sold us a nice rectangular down sleeping bag and a sheet that zipped to it which securely held two pads. The bag has multiple zippers so that one can open one or both foot//feet section(s) and stick a foot out. That's what we've used since then, except that we lost the sheet and had to make our own replacement and we've had the bag relofted once. (Marmot did it for free.)

One big advantage is that this one bag packs nice and small, which saves room. You really don't need as much bag when there's two bodies in it. There's also a small weight savings in having only one bag (the sheet weighs about nothing). As mentioned by others, there are additional benefits to such a sleeping arrangement.